Optical fiber sensors allow simultaneous measurements of a plurality of parameters, such as the combination of axial strain and temperature. It has proven to be difficult to make optical fibers which are insensitive to temperature, or which are self-compensating. The general problem in the prior art, then, is that of independently sensing the strain, or of separating the strain from temperature effects. In general, sensor systems capable of separation of the two effects have used sensors in the form of (a) a combination of interferometric and polarimetric sensors, (b) two interferometers along independent eigen axes, or (c) two Bragg gratings on a high-index optical fiber. These approaches to sensing have been found to be subject to large errors and instabilities, because the defining equations are not sufficiently independent.
Air gap Fabry-Perot sensors overcome many of the drawbacks of the abovementioned sensors. FIG. 1a illustrates a conceptual Fabry-Perot etalon 10, such as that described in U.S. Pat. No. 5,276,501, issued Jan. 4, 1994, in the name of McClintock et al. Etalon 10 includes a lead-in optical fiber 12 with an end or face 14, and another optical fiber 16 with an end or face 18, with a gap 20 therebetween. When light flows between fibers 12 and 16, Fresnel reflections occur at the ends 14 and 18, due to the change in index of refraction between the air of the gap and that of the optical fibers. The Fresnel reflections are partial, in that only a portion of the incident light is reflected, and another portion is transmitted past the glass/air interface. The reflection from end 18 of fiber 16 is delayed or phase-shifted relative to the reflection from end 14, because of the length S of gap 20. The reflection from face 18 is delayed by the round-trip transit time through the gap, corresponding to a distance 2S. A similar arrangement, using gradient-index (GRIN) lenses to reduce losses, is described in an article entitled FIBER COUPLING USING GRADED-INDEX ROD LENSES, by Palais, published in APPLIED OPTICS magazine, Vol 19, No. 12, Jun. 15, 1980. Those skilled in the art know that the length of the cavity, and changes in the cavity length, can be measured by various readout arrangements. As illustrated in FIG. 1a, fibers 12 and 16 are unsupported.
FIG. 1b illustrates a prior-art in-line Fabry-Perot etalon (EFPI) sensor 22 in which the fibers are held in position by an in-line tube, to define an intrinsic Fabry-Perot etalon (ILFE). In FIG. 1b, ILFE 22 is sensitive to strain in the axial direction. In FIG. 1b, glass tube 24 has an outer diameter substantially equal to that of the lead-in fiber 12 and the lead-out fiber 16, and also defines a bore 26. Partial reflections take place at that part of face 14 of fiber 12 which is within the bore, because of the glass/air interface within the bore. A similar reflection takes place at face 18 of fiber 16.
FIG. 1c illustrates a prior-art etalon, as described, for example, in U.S. Pat. No. 5,359,405, issued Oct. 25, 1994 in the name of Andrews, in which the fibers are held in position within the bore of a tube to define an extrinsic Fabry-Perot etalon (EFPI). Such a structure is termed a "Fizeau" sensor in U.S. Pat. No. 5,301,001, issued Apr. 5, 1994, in the name of Murphy et al. In FIG. 1c, a glass tube 30 has a bore 32 and first and second ends 34 and 36, respectively. Face 14 of lead-in optical fiber 12 is within bore 32 of tube 30, together with a portion of fiber 12, and face 18 of optical fiber 16 is also within bore 32. Faces 14 and 18 together define a multiple reflection etalon similar to that of FIG. 1a. As described in the Andrews patent, the fiber faces 14 and 18 have reflection magnitudes of about 4% if uncoated, and may be coated to change the reflection magnitude. The support of the fibers 12 and 16 within the bore 32 of tube 30 allows full utilization of the area of reflective surfaces 14 and 18. Fibers 12 and 16 are fastened to tube 30, as by fusion welds, at locations 38 and 40, which are near tube ends 36 and 38, respectively.
The arrangement of FIG. 1c allows strains in external tube 30 as a result of externally applied stresses to be effectively magnified, by comparison with the arrangement of FIG. 1b. More particularly, external force or stress applied axially to the tube 24 of FIG. 1b, as, for example by imbedding the tube in a structure to be measured, is applied across the length S of tube, and the resulting strain (deformation in the form of stretch or compression) is some constant K times S, or KS, where K depends upon the nature of the glass tube 30. This strain is measured across gap distance S in the arrangement of FIG. 1b. In the arrangement of FIG. 1c, on the other hand, the length of the tube 30 is Y times S, so the force tending to cause deformation is applied across a longer portion of the tube, and the resulting strain is correspondingly longer, namely Y times KS, or YKS, which is Y times greater than in the case of FIG. 1b. The greater strain, however, appears across the same gap length S, because fibers 12 and 16 are free to move within tube bore 32, being constrained only at or near the ends 34 and 36 of the tube 30. Consequently, a strain Y times that of FIG. 1b appears across the same gap length S in the arrangement of FIG. 1c, with the result of higher sensitivity in the form of more strain or motion of a given gap length for the arrangement of FIG. 1c by comparison with that of FIG. 1b. Optical fiber 12 is a single-mode fiber, and fiber 16 is either a single-mode or a multimode fiber.
When sensor 8 of FIGS. 1b or 1c are illuminated from one end, two Fresnel reflections are created at the reflective fiber faces. These two reflections return toward the source, and can be separated from the incident light by means of a directional coupler. The separated reflections can be evaluated to determine the dimensions S of the gap. A wavelength tuning method is described in the abovementioned Andrews patent.
As mentioned, sensors 16 of FIG. 1b and 28 of FIG. 1c, when connected to a readout system, are capable of measuring strain. The strain is manifested as a change in dimension S of cavity 20. Strain is defined as change in length divided by length, or .DELTA.L/L, and may be caused by either a physical force applied to the ends of the tube, or it may be caused by the temperature coefficient of physical expansion of the support tube 24 or 30. Without knowing the actual temperature of the sensor, then, it may not be possible to know the amount of change of dimension of the cavity which is due to external forces, rather than to temperature changes. The use of multiple measurement sensors in the same physical structure tends to weaken the structure, and the presence of the one sensor near another can itself affect the desired measurements. Improved sensors are desired.